Eccentric tensioners may be categorized as single eccentric, one pivot arm, or double eccentric, two pivot arms, each having bores and bearing surfaces. The center of rotation of the bore(s) is eccentrically displaced from the center of rotation of the bearing. This eccentric displacement, coupled with a spring force, imparts a moment which can be translated in to a force to allow tensioning of a drive belt.
The design principle for a double eccentric tensioner is for one pivot arm, the adjustor pivot arm, to be such that it is able to compensate for build tolerances. This pivot arm is locked in position as part of the installation procedure. The second pivot arm, the working pivot arm, controls the belt tension. This pivot arm is free to move, within limits, during operation. The limits are set to avoid conditions which could be detrimental to the drive system or any component within the drive system.
The combined length of these pivot arms is dependant on the geometry of the drive, the tolerance stack up, the wrap angle, the drive dynamics and the tension at which the drive is required to operate.
The tensioner baseplate need also be at a known orientation to the drive belt. This is normally achieved by means of incorporation of a tang in the baseplate design which then cooperatively locates in a recess in a predetermined position on the engine block. This can also be achieved by other means as well known in the art.
Prior to installation, there is normally no need to preload the spring on this type of tensioner
With respect to the single eccentric tensioner, it comprises only one pivot arm, the working pivot arm, which compensates for build tolerances and controls the belt tension within the required specification. The pivot arm is free to pivot, within travel limits, during normal operating conditions. The travel limits are predetermined to avoid conditions which could be detrimental to the drive system or any component within the drive system.
The length of the pivot arm is dependant on the geometry of the drive, the tolerance stack up, the wrap angle, the drive dynamics and the tension at which the drive is required to operate.
This type of tensioner may or may not include the need for the tensioner baseplate to be at a known orientation to the drive belt. If required it can be achieved in the same manner as for the double eccentric tensioner.
Prior to installation on an engine, the spring for the single eccentric tensioner is “preloaded” and a load pin, or other similar device, temporarily holds the tensioner body in a predetermined position relative to the baseplate. The position is calculated to give sufficient clearance from the belt to allow the tensioner to be mounted on the engine during the tensioner/drive installation procedure. The tensioner spring is under compression in this condition. Removal of this load pin, or other such device, during installation, releases the spring thereby tensioning the belt.
For both types of tensioners a typical installation procedure requires the engine crankshaft to be set at a predetermined rotational angle. The camshaft(s) are also set and locked by some means at a known relative angle to the crankshaft. These angles are defined by the engine designer.
The belt drive system can then be mounted on the engine. Components which require to be timed relative to the crankshaft should be locked in the “timed” position at this point.
For a double eccentric tensioner, the tensioner is then mounted in the desired orientation. The fixing bolt is inserted through the tensioner to the engine mounting point and semi tightened. The tensioner is then rotated, initially using the adjustor pivot arm to take up slack in the drive, and then using the working pivot arm to tension the belt. The fixing bolt is then tightened thereby locking the adjuster pivot arm.
The tensioner travel limits are pre-positioned relative to the baseplate. The operating position is determined by means of a pointer on the pivot arm which rotates to a reference set point during installation, thereby indicating that the correct tension level has been achieved.
For a prior art single eccentric tensioner, if required, the tensioner is mounted in the correct orientation. The fixing bolt is inserted through the tensioner in to the engine fixing point and tightened. The load pin, or other such device, is then released thereby simultaneously taking up slack in the drive and achieving the correct belt tension level.
The tensioner design may permit the tensioner travel limits to be correctly positioned during the tensioning process by some mechanical means or some other manual adjustment may be necessary to achieve this.
All pulleys are then fully tightened. For a double eccentric tensioner belt drive system, the engine is rotated through 360 degrees, or a multiple thereof, and the tensioner pivot arm pointer position then rechecked.
Representative of the art is U.S. Pat. No. 4,832,665 to Kadota (1989) which discloses a tensioner comprising a moveable eccentric member rotatably provided about a fixed member. A spring is provided between the stationary portion and the moveable eccentric member for biasing the eccentric member in a direction to press an idler into contact with a driving member.
What is needed is an eccentric pivot arm tensioner having cooperating members for automatic release of a preloaded pivot arm torsion spring and substantially simultaneous automatic adjustment of pivot arm travel range limits during installation. The present invention meets this need.